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Euclidean, hyperbolic, discrete, convex, coarse geometry, metric spaces, comparisons in Riemannian geometry, symmetric spaces.

8 votes
Accepted

Determining the stretch of a cluster of points

There are many roundness measures that have been explored for different applications, which may give you ideas. A good source for roundness in image processing is this paper, which analyzes and compar …
Joseph O'Rourke's user avatar
3 votes
Accepted

Alexandrov curvature of a compact length space

From "A.D. Alexandrov spaces with curvature bounded below", Burago, Y. and Gromov, M. and Perel'man, G., Russian Mathematical Surveys, 47, 1992, p.5: A locally complete space $Μ$ with intrinsic me …
Joseph O'Rourke's user avatar
2 votes

Smallest ball containing the intersection of a family of balls

Even though this doesn't answer any of your questions, it might help to look at the literature on the Kneser–Poulsen conjecture: If a finite set of balls is repositioned so that the distance between t …
Joseph O'Rourke's user avatar
12 votes

Feasibility of a list of prescribed distances in R^3

What did not seem to be mentioned in this old thread is that this is a well-studied problem that goes under the name distance geometry, which Wikipedia defines as "the characterization and study of se …
Joseph O'Rourke's user avatar
7 votes

Advanced view of the napkin ring problem?

This is my attempt to understand David Eppstein's construction. The green segment is the "stick connecting the center of the sphere to a point halfway from top to bottom on the inner surface of the ho …
Joseph O'Rourke's user avatar
1 vote

Surface area of superellipsoid (dice)

Here is an image of Will Jagy's colloidal dice shape:      
Joseph O'Rourke's user avatar
3 votes

Some questions about polyhedra in 3-dimensional Euclidean space, E3

Concerning your last question (conditions for a set of points to be realized as the vertices of a polyhedron), it was established by Branko Grünbaum that every non-coplanar finite set of four or more …
Joseph O'Rourke's user avatar
6 votes
Accepted

Smallest value of largest angle in finite planar configurations

The key bound is $(1 - 1/n) \pi$, due to Erdős and Szekeres:            The above is an excerpt from this paper:            The Erdős-Szekeres result is in their 1961 paper, "On some extremum pr …
Joseph O'Rourke's user avatar
1 vote

A question about maximizing the volume of a particular kind of convex set

It seems you could curve the base of the cone, making it a sector of a sphere of radius $r > d$ slightly larger than $d$, centered at a point $c$ directly above the apex of the cone: (Arrows indicate …
Joseph O'Rourke's user avatar
3 votes

Is there always a maximum anti-rectangle with a corner square?

Could you please clarify your definitions via the example polygon below?     Is $\{a,b,c,1,12\}$ an antirectangle of size $5$? Is this a maximal antirectangle? What is a largest antirectangle that inc …
Joseph O'Rourke's user avatar
4 votes
Accepted

Is there interesting structure in the space of Voronoi diagrams?

Although I am not sure I fully understand the question, perhaps this attempt at a partial answer will clarify matters. First, it is known that every tree of at least one edge and with no vertices of …
Joseph O'Rourke's user avatar
7 votes
0 answers
250 views

Equiareal shapes in $\mathbb{R}^d$

There was quite a bit of work on the so-called equichordal problem throughout the 20th century, to decide if some plane convex curve could have two equichordal points. A point is equichordal for a clo …
Joseph O'Rourke's user avatar
3 votes
Accepted

Mahler Volume of the Snub 24-Cell

This is just a comment embellished with an image. It seems that the paper you cite, in its earlier arXiv version, Mehmet Koca, Mudhahir Al-Ajmi, Nazife Ozdes Koca. "Quaternionic Representation o …
Joseph O'Rourke's user avatar
4 votes
1 answer
787 views

When does the intersection of cylinders produce a ball?

Suppose one intersects unit-radius solid cylinders in $\mathbb{R}^3$, with each cylinder axis passing through the origin. For example, two such cylinders produce the Steinmetz solid. But if we imagin …
Joseph O'Rourke's user avatar
2 votes

Peeling a polygonal vegetable

This subverts your intent, but in this example, $P(t)$ cannot remain connected as it must jump to other hole, and then outside the pulp.           So perhaps you should assume that the pulp polygon …
Joseph O'Rourke's user avatar

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